1. Field of Invention
The invention describes a new method for forming glass compositions having low melting points and low thermal expansion. Described herein are various materials, seal designs, and geometries of seals based on glass powders. The glass composition of the invention includes B2O3, Na2O, Al2O3, Li2O, TiO2, ZnO, Ta2O5, Nb2O3, BaO, ZrO2 and SiO2, and is devoid of lead, bismuth, and vanadium.
2. Description of Related Art
Conventional formation of hermetic seals for electronic devices such as thin layer solar cells or OLEDs involves the application of organic pastes or tapes at the edges of the device followed by curing cycles that are induced thermally, with ultraviolet light or chemically. Organic sealants are not truly hermetic and sometimes contain getters to mitigate their non-hermeticity. Truly hermetic seals can be made from glass based systems by printing and firing a paste composition including a glass fit and organic solvent/binder system. Drawbacks of such systems include the need to fire at extremely high temperatures, e.g., 800° C., and the problem of entrapment of organic residue and incomplete burnout of the binder system (such as ethyl cellulose) within the hermetic seal. Such entrapment causes undesired contamination of sealed active sealed, such as solar cells or OLEDs, within the hermetic seal.
Conventional glass sealing methods and compositions may have too high or too low an expansion (CTE) relative to a substrate glass plate and also too high a melting point. A melt point that is too high may result in warpage of glass substrates or damage to the seal itself.
The present invention relates to a low melting glass, a sealing composition and a sealing paste, and more particularly to those which contains no lead, no vanadium and no bismuth component and are used for manufacturing of a flat panel display such as plasma display panel (PDP), liquid crystal display (LCD) or light emitting diode (LED).
Conventionally, a sealing composition used to seal the periphery of PDP, LCD or LED was generally a lead-based material or the like. Recently the market, under environmental pressures, has demanded a composition that does not contain hazardous components such as lead, bismuth or vanadium, and can seal at a low temperature.
As a low melting glass not containing lead or vanadium component, there are known phosphate glass (U.S. Pat. No. 5,256,604) and bismuth glass (U.S. Pat. No. 5,733,828, U.S. Pat. No. 5,346,863), among which bismuth glass attracts much attention because it can be fired at a low temperature and exhibits adequate chemical endurance.
Accordingly improvements are sought in sealing processes.